The epidermal growth factor receptor (EGFR) is a cell surface transmembrane receptor of the HER/ErbB receptor family that transmits signals (including mitogenic signals that drive cell proliferation) to the interior of a cell when activated, typically by the binding of any of a number of extracellular ligands such as epidermal growth factor (EGF). EGFR ligands vary in their affinity for EGFR and are categorized as either high- or low-affinity ligands. It is thought that the high- and low-affinity interactions between EGFR and its ligands activate different signaling pathways. This signal transmission occurs through a cascade of intracellular events beginning with protein phosphorylation mediated by receptor tyrosine kinase activity. EGFR has proven a responsive target for anti-proliferative (e.g., anti-cancer) drugs, including “small molecule” tyrosine kinase inhibitor drugs (typically no larger than 700-900 AMU) that may be orally administered as well as monoclonal antibody based drugs that specifically bind to the extracellular domain of EGFR. EGFR-targeted monoclonal antibodies are not always effective against EGFR-expressing tumors. One approach taken with the aim of improving anti-EGFR antibody efficacy has been to develop mixtures of anti-EGFR monoclonal antibodies (i.e., oligoclonal antibodies) targeted to different sites (epitopes) on to the extracellular domain of EGFR. See, e.g., PCT Int. Pub. No. WO/2011/140254 and U.S. Pat. No. 7,887,805. These developments have created a need to enable the identification of cancer patients whose tumors have characteristics rendering them unresponsive to monoclonal anti-EGFR antibodies so that such patients may receive effective treatment via administration of oligoclonal anti-EGFR antibodies. The present disclosure answers this need and provides other benefits.